The present invention relates to a transmitter for a tire state monitoring apparatus, and more particularly, to a transmitter connected to a wheel, to which a tire is attached, to transmit information related with tire air pressure to a receiver arranged in a vehicle.
In recent years, for safety reasons, the employment of a wireless tire monitoring apparatus is becoming popular which checks the state of vehicle tires from inside a vehicle when the vehicle is being driven.
Referring to FIG. 1, a wireless tire state monitoring apparatus includes a transmitter 50 to transmit information related with tire air pressure to a receiver (not shown), which is installed in the passenger compartment. The transmitter 50 includes a box-like casing 51 and a valve stem 52, which extends integrally from the casing 51. The casing 51 accommodates a circuit board (not shown) and a battery (not shown). Pressure detecting devices and circuit devices for processing various signals are mounted on the circuit board. The battery supplies the devices with power. A vent hole (not shown) extends through the casing 51.
As shown in FIG. 2, the transmitter 50 is connected to the wall defining a valve hole 63 of a tire wheel 62 to arrange the casing 51 in a tire 61. Air is charged into the tire 61 from the valve stem 52 and through the vent hole.
To decrease the weight of the transmitter 50, the casing 51 is formed from resin. When the vehicle is being driven, centrifugal force is applied to the transmitter. The centrifugal force varies in accordance with the diameter of the tire wheel 62, the outer diameter of the tire 61, and the velocity of the vehicle. For example, the centrifugal force reaches about 1,500 G at 300 km/h. The centrifugal force acts to deform the casing 51 of the transmitter 50 such that the casing 51 moves away from the tire wheel 62. As a result, in the transmitter 50, when the casing 51, which is formed integrally with the valve stem 52, is deformed regardless of its resiliency, stress is produced in the circuit board accommodated in the casing 51. Such mechanical stress may lead to deficient connections.
Referring to FIG. 2, when the tire 61 is attached to or removed from the tire wheel 62, the bead 61a of the tire 61 passes by the casing 51 of the transmitter 50. As the bead 61a passes by the casing 51, the bead 61a pushes the casing 51 of the transmitter 50 toward the tire wheel 62. When the angle of the axis of the valve hole 63 in the tire wheel 62 relative to the axis of the tire wheel 62 (hereinafter referred to as valve hole angle) is large, the casing 51 is separated from the drop center 62a of the tire wheel 62. In such a case, it is difficult for the bead 61a of the tire 61 to smoothly pass by the casing 51 of the transmitter 50, and the force applied to the casing 51 by the bead 61a increases. This increases the possibility of an abnormality occurring in the circuit board accommodated in the casing 51.
To prevent such problems in the prior art, the transmitter 50 is formed so that the inclination of the casing 51 relative to the axis of the valve stem 52 is greater than the valve hole angle.
However, the valve hole angle is only required to be within a predetermined range (15 degrees to 25 degrees) and the location of the valve hole 63 differs slightly between tire wheels 62. Further, the inclination of the rim 62b relative to the drop center 62a of the tire wheel 62 differs between products depending on the design of the wheel. Thus, the inclination angle of the casing 51 should be fixed for each product to guarantee optimal connection to the wheel 62. However, this would increase the cost for manufacturing the transmitter 50.